The Prader-Willi Syndrome (PWS) and Miller-Dieker Syndrome (MDS) are both associated with mental retardation and dysmorphic features. Recently, we have shown that both syndromes are caused by deletions of chromosomes 15 and 17, respectively. In both, the deletion is extremely small, requiring high-resolution techniques for detection. Even then, a percentage of patients appear to have normal karyotypes. Since submicroscopic deletions may be present, more sensitive and reliable diagnostic techniques are needed for these two disorders. Analysis at a molecular level would be beneficial, but few cloned genes or anonymous DNA segments have been mapped to chromosomes 15 or 17 and none falls within the critical region of these disorders. Recently, flow-sorted libraries for chromosomes 15 and 17 have become available from the Los Alamos and Lawrence Livermore National Laboratories. Additionally, a partial chromosome 15 library containing only the PWS critical region (q11-q13) is being constructed elsewhere. These materials make it possible to do detailed molecular analysis of these chromosomes, including dissection of the critical regions for PWS and MDS and the development of linkage maps for each chromosome. Using published cloned genes, arbitrary DNA segments, and flow-sorted libraries for chromosomes 15 and 17, the goals of the project are: 1) Molecular dissection of PWS and MDS. Identify DNA probes which fall into the critical region of deletion in PWS and MDS to be used as diagnostic tools in conjunction with high-resolution cytogenetic analysis. These probes will be tested against clinically affected patients with apparently normal karyotypes in order to determine whether submicroscopic deletions exist. For PWS, these probes will be useful in determining whether the breakpoints of all interstitial deletion patients are consistent, suggesting a "hot spot" for chromosomal rearrangement. These probes will help determine whether PWS patients with duplications and extra marker chromosomes have the same DNA segments affected. Finally, polymorphic probes within the PWS and MDS regions will be useful for determining the parental origin of de novo chromosome abnormalities. 2)\Development of linkage maps for chromosomes 15 and 17. Regionally map 20-30 DNA probes to specific regions of chromosomes 15 and 17 by somatic cell hybrid analysis using rearranged chromosomes 15 and 17, dosage analysis using unbalanced patient cell lines, and in situ hybridization. A search for restriction fragment length polymorphisms will be conducted to establish a genetic linkage map for both chromosomes which will be useful in the analysis of other human genetic diseases. Linkage analysis may be employed to determine the recombination distance between probes and their order on the chromosome.